The present invention relates to a novel zinc borate, a method of preparing the same and use thereof. More specifically, the invention relates to a novel zinc borate having a particular crystallite size and containing very little sodium components and to a method of preparing the same.
In recent years, resin molded articles have been used in a wide variety of fields, and it has been desired to impart flame-retarding property to these resins. It has further been desired that the resin molded articles such as building materials produce smoke in suppressed amounts so will not to hinder people from taking refuge in case fire has broken.
It has long been known that a zinc borate exhibits excellent flame-retarding property. For example, Japanese Examined Patent Publication (Kokoku) No. 67363/1981 teaches blending a zinc borate with a metal oxide such as Al2O3, SiO2, Sb2O3, ZnO or ZrO2.
Japanese Unexamined Patent Publication (Kokai) No. 137988/1988 filed by the present applicant discloses a flame-retarding composition obtained by treating the surfaces of a zinc borate particles with an oxide, a hydroxide or a basic carbonate of an alkaline earth metal of an amount of 1 to 40% by weight per the whole weight.
A method of preparing a zinc borate has also been long known. For example, Comparative Example 1 of Japanese Examined Patent Publication (Kokoku) No. 20902/1971 teaches the preparation of a compound of the formula,
2ZnO.3B2O3.9H2O 
by mixing a combination of borax pentahydrate and boric acid with a zinc oxide in an aqueous solution of sulfuric acid, followed by the addition of seed crystals of a zinc borate.
However, the conventional zinc borate assumes the form of particles of indefinite shapes having relatively large diameters in which fine primary particles are coagulated in a random fashion and densely, and poorly disperses in the resin, fails to impart luster on the surfaces of the molded articles of a resin blended therewith, and offers poor appearance and decreased commercial value.
Further, the known zinc borate has been synthesized in an aqueous medium containing sodium salts and, hence, contains sodium components as impurities in considerably large amounts. When the plastic molded articles for electric and electronic parts are blended with such a zinc borate for a purpose of imparting flame-retarding property, it is probable that the electric properties are deteriorated as represented by poor insulation and dielectric breakdown after the use for extended periods of time being affected by humidity.
It is, therefore, an object of the present invention to provide a zinc borate having a particular crystallite size and containing very little sodium components, and a method of preparing the same.
Another object of the present invention is to provide a zinc borate which excellently disperses in a resin, markedly improves smoothness, luster and appearance of the surfaces of a resin blended therewith, and maintains, on an excellent level, the electric properties of the resin blended therewith as a flame-retarding agent or a smoke-suppressing agent, and a method of preparing the same.
According to the present invention, there is provided a zinc borate having a chemical composition represented by the following formula (1),
2ZnO.mB2O3.XH2Oxe2x80x83xe2x80x83(1) 
wherein m is a number of from 2.8 to 3.2, and x is a number of not larger than 4,
and having a crystallite size of not smaller than 40 nm as found from diffraction peaks of indexes of planes of (020), (101) and (200) in the X-ray diffraction (Cu-kxcex1) and containing sodium components in amounts of not larger than 100 ppm as measured by the atomic absorptiometric method.
In the zinc borate according to the present invention, it is desired that the individual particles are independent rhombic hexahedrons, the length of a side of each particle lying in a range of from 0.3 to 7.0 xcexcm as measured by a scanning-type electron microphotograph.
In the zinc borate according to the present invention, it is desired that a product of crystallite sizes as found from the diffraction peaks of indexes of planes (020), (101) and (200) is not smaller than 200,000 nm3, and that a volume-based median diameter as found by a laser diffraction method is in a range of from 1.0 to 6.0 xcexcm.
The invention further provides a method of preparing a zinc borate by forming fine crystals of a zinc borate by reacting a zinc flower and a boric acid at a substantially stoichiometric ratio at a relatively low temperature, effecting the aging as required and, then, maintaining the reaction system at a relatively high temperature to grow the crystals.
According to the present invention, further, there are provided a flame-retarding agent or a flame-retarding assistant, a smoke-suppressing agent, an antibacterial agent and a water glass-curing agent comprising the above zinc borate.